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PORuby IO.popen with "-" , what happens under the hood?
 

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    <p>I'm trying to understand IO.popen when its command is <code>"-"</code> which starts a new Ruby interpreter.</p> <p>There is not much material about this subject, and I'm getting slowly through them, mainly because of me as I only code for fun.</p> <p>As far as I have understood when <code>IO.popen("-", "w+") {|f| ...}</code> is invoked - that's with a block - that block will be run by <em>both</em> the parent and the child process. The difference is that the parent process will get an IO object as a result, but the child gets only a Nil. That's easy, I need to check <code>|f|</code> in the block and when it is Nil, execution is in the child process, when it is not nil, execution is in the parent. So I have to write both code for parent and child separated by <code>if</code>. </p> <p>This time it helps me to understand the problem, that <em>the block is part of the IO.popen command</em>.</p> <p>I have this code:</p> <pre><code>pipe = IO.popen("-","w+") # puts "This line will break functionality if uncommented" if pipe != nil then pipe.puts "PID: #{Process.pid}" $stderr.puts "Parent from child: #{pipe.gets.chomp}" else $stderr.puts "Child PID: #{Process.pid} and Parent #{gets.chomp}" puts "M'kay" end </code></pre> <h2>Questions:</h2> <ul> <li>What decides which process runs first? If they were to append a file would it be vulnerable to race condition?</li> <li>Why the 2nd line breaks the code? The <code>pipe = IO.popen...</code> command shouldn't be related to the <code>if..else..end</code> block, yet they are. For me <code>pipe</code> is a file handle (like in old Turbo Pascal) which is first definded somewhere, then manipulated elsewhere.</li> </ul>
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